Electrodeless discharge lamps have been the subject of significant development efforts lately because of the known energy efficiency and long life characteristics. Such development efforts have been directed to the task of producing such a lamp at a reasonable cost to consumers and one which exhibited performance characteristics that were well within requirements relating to EMI (electro-magnetic interference). Examples of such electrodeless lamps can be found in U.S. Pat. Nos. 5,412,280 and 5,412,288 both of which are assigned to the same assignee as the present invention. It can be seen that this lamp is a low pressure discharge lamp which is configured in the form of a fluorescent reflector lamp and as such, includes a phosphor coating for generating a white light output for commercial and residential lighting applications. It is because of these long life and energy efficiency characteristics that a discharge lamp is being proposed for an automotive application where it is necessary to provide a flashing operation. However, although effective for providing an efficient, long-life lighting product, modifying such a lamp to provide a red light output would require the use of a red phosphor material. It is impractical to provide a red phosphor to achieve the desired red light output since a red phosphor electrodeless fluorescent lamp would have low efficacy values at low temperatures, such low temperatures being an unavoidable condition for automotive applications.
One way to provide a new type of red tail light/signalling lamp for automotive applications would be to utilize a neon fill, i.e. the new lamp is substantially void of phosphor in conjunction with known lamp types such as a standard electroded discharge lamp. Such a neon lamp would be an improvement for automotive lighting designers as compared to a standard halogen, incandescent lamp because of the faster turn on times that could be achieved for a discharge lamp in comparison to an incandescent lamp. However, one problem that arises with the typical implementation of neon in a gas discharge lamp is that a cold cathode system (that is, one which does not preheat the cathodes for easier starting) must use high voltages to insure reasonable efficiency. In an automotive application, it would not be appropriate to use such high voltages at a location near gasoline. Additionally, it has been measured that the luminous efficacy of a cold cathode neon lamp is on the order of approximately 3 to 7 lumens per watt (LPWs). If a hot cathode neon lamp were to be utilized, there is typically a reduction in the life of such lamp as well as an increase in the cost of manufacture. Moreover, it has been measured that such a hot cathode neon discharge light source would only achieve a luminous efficacy of approximately 15 LPWs. Accordingly, it would be advantageous to achieve a light source which could provide the necessary red color light output without the cost and disadvantages of either a cold or a hot cathode discharge lamp arrangement and yet could also achieve a luminous efficacy of greater than approximately 20 LPWs.
An additional problem that would be experienced because of the presence of electrodes in a standard electroded discharge lamp is the fact that for automotive signalling purposes, it is necessary to flash the light in an on and off condition as for instance, for a turn signal. It is known that for an electroded discharge lamp, the long life characteristics are at least partially due to a condition whereby the lamp can be turned on and left on for a significant period of time, when it is necessary to utilize such a lamp in a flashing manner, electrode degradation, typically tungsten sputtering, can occur. This electrode degradation is a life-limiting mechanism for electroded lamps. Accordingly, it would be advantageous if a light source could be provided that had the long life and energy efficiency characteristics of a discharge light source but yet could be utilized in an automotive application requiring signalling and tail light performance.